The present invention relates to a method of controlling the heating of an oxygen sensor fitted to the exhaust system of an internal combustion engine for the purpose of controlling air-fuel mixture air/fuel ratio, and to a system for practicing the method. More particularly, the present invention relates to such a method and device for oxygen sensor heating control which limit the current passing through and the power dissipated in a resistive electrical heater element of the oxygen sensor, so as to ensure that the heater element is not damaged by too great a thermal shock.
It is known to fit an oxygen sensor to the exhaust system of an internal combustion engine. Such an oxygen sensor typically comprises a solid electrolyte or semiconductor, and varies a generated current or resistance in response to the concentration of oxygen in the exhaust gases of the engine. This electrical signal is fed to a control device which controls the amount of fuel provided to the engine in relation to the amount of air sucked thereinto, and is used for controlling the air/fuel ratio of the air-fuel mixture supplied to the engine by a feedback process. Various such forms of control device, which practice various methods of air-fuel mixture rtio control, are per se known.
The output of the sensor element of such an oxygen sensor varies with temperature, and, particularly when the air/fuel ratio is weak and is in the range of 14.5 to 25, in order for the sensor element to accurately measure the oxygen concentration, said sensor element must be maintained at a temperature higher than a certain critical minimum active temperature. This maintenance of the temperature of the sensor element can be done by using a heater, and oxygen sensors with sensor element heaters have already been proposed, along with methods for operation of such heaters; for example in Japanese Patent Application No. 53-78476, which has been published as Japanese Patent Publication No. 54-13396. Further, in Japanese Patent Application No. 53-83120, which has been published as Japanese Patent Publication No. 54-21393, there has been proposed a method and a system for control of the electrical power supplied to such an oxygen sensor element heater, in which the power is varies as a function of intake manifold pressure, of throttle opening, and of engine revolution speed, so as to ensure that the oxygen sensor element is kept at a temperature no lower than its minimum active temperature.
The sensor element of such an oxygen sensor fitted to an exhaust system is of course heated up by the exhaust gases in the exhaust system, so the effect of a heater for the sensor element must be controlled to take account of the temperature of these exhaust gases. Now, in an internal combustion engine which is controlled by a throttle valve, the exhaust temperature is largely determined by the amount of air-fuel mixture supplied per engine piston stroke and by engine revolution speed, and if the air/fuel ratio of the air-fuel mixture is constant the amount of such mixture supplied is proportional to the rate of intake air flow. Therefore, in the above mentioned patent applications, the above are used as parameters, and the supply of electricity to the sensor element heater is varied depending on the engine load and the engine revolution speed. Thus, the exhaust temperature is considered to depend on the engine intake flow and engine revolution speed, and the values are determined experimentally in advance with reasonable accuracy. This method and system are adequate to keep the temperature of the sensor element of the oxygen sensor reasonably constant regardless of engine operational conditions.
The sensor element of such an oxygen sensor fitted to an exhaust system is of course at a temperature substantially the same as that of the engine as a whole, when the engine has not been running for any substantial time. Now, the heater element is typically of a pure resistive load type, using the Joule heating phenomenon for producing heating power, and typically such a heater element has a resistance which increases as its temperature increases. In other words, since the heater element is usually designed in view of its use when at its normal operating temperature, its resistance when cold, i.e. when the engine is cold, is relatively rather low. Thus, when the engine is first started up from cold, there is a risk that initially the current through the heater element may be rather large, due to its low resistance, and that thus an unduly high amount of electrical power may be initially dissipated in the heater. If this happens, the resultant rapid rate of heating up of the heater element may cause a thermal shock which is bad for the endurance of the heater element. In the case of, for example, a platinum resistor on an alumina mount, which is a fairly typical type of heater element for this sort of application, sudden rises in temperature caused by such unduly high power dissipation in the heater element may cause thermal stress and overheating of the material of the heater element, which can lead to agglomeration of this heater material which can in its turn cause a discontinuity to arise in the circuit; or thermal stress on the mount of the heater element may cause it to be broken. All these problems affect the reliability of the heater element.
In Japanese Patent Application No. 56-203678, which has been published as Japanese Patent Publication No. 58-105056, the suggestion has been made to limit the initial voltage supplied to the heater element, so as to restrict the power dissipation therein in the initial phase of engine operation. However, in this prior art, this is suggested to be done from the time that the heater is switched on, which is determined from the state of the switch controlling the heater, and thus the voltage supplied to the heater element is restricted in all cases irrespective of the actual temperature of or of the resistance of the heater element, and in every instance of starting up, irrespective of whether the engine is cold or warm when it is being started up, and accordingly sometimes the heater element voltage is restricted when this is not necessary, as during warm restarting. This is means that the restriction of the heater voltage for the purpose of limiting surges cannot be kept to a minimum.
An alternative solution, proposed in Japanese Patent Application No. 56-181531, which has been published as Japanese Patent Publication No. 58-83251, is to delay energizing the heater element until the oxygen sensor and its heater have been somewhat heated up by the exhaust gases of the engine, so that the resistance of the heater element has been somewhat increased. But this involves waiting for the exhaust gases to warm up the heater element, and delays use of the heater element for properly warming the oxygen sensor element, which delays starting the feedback control of the air/fuel ratio of the air-fuel mixture.